The two signal model and the concept of costimulation are well established in T cell regulation. Costimulation through CD28 can have a dramatic impact on many aspects of T cell activation, survival, tolerance, and differentiation. However, in spite of considerable interest and effort, the mechanisms of TCR and CD28 signal integration are not well understood. Our recent data indicate a novel site of regulation between TCR and CD28 signaling. We have found that TCR signaling can enhance CD28 ligand binding. Based on our preliminary data we have developed a model that TCR signaling induces a change in the orientation of the CD28 extracellular domains, allowing for bivalent binding. This increase in valency then accounts for the ability of TCR to enhance CD28 ligand binding. Although traditionally we think of CD28 as a modifier of TCR signaling, these results create a new paradigm for T cell activation, showing that TCR may also regulate CD28 function. In combination with the established role for TCR signaling in integrin activation, our results indicate that TCR may coordinate ligand binding for both the major costimulatory molecule (CD28) and the major adhesion molecule (LFA-1) during immunological synapse formation and T cell activation. This discovery of a novel regulatory mechanism in T cell activation identifies a new potential target for immunotherapy. With this goal in mind, we have developed experimental approaches that will directly test some of the predictions of this model and will use these reagents to identify new small molecules that can specifically modulate CD28 function, either as agonists, promoting effective T cell responses, or antagonists, inhibiting T cel responses. We will achieve these goals through two Specific Aims. 1. Generate structural mutations in the lumenal domain of CD28 that either lock CD28 in the low avidity form or stabilize the high avidity conformation. 2. Identify small molecule agonists and antagonists.